This application claims priority based on the U.S. Provisional Application 61/076,358 filed on Jun. 27, 2008, which is incorporated herein by reference in its entirety.
The present invention is related to a liquid crystal (LC) device and methods thereof. It finds particular application in conjunction with liquid crystal display (LCD), STN and SmC* devices; and sensors for electric field, magnetic field, thermal field, chemical species, biochemical species, biological species, or any combination thereof, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Benchtop spectroscopic methods such as mass spectroscopy, gas chromatography, and infrared spectroscopy can give precise and quantitative detection. However, these methods use bulky instruments which are not portable, and it is complex to provide real-time detection. A liquid crystal sensor is another option for chemical detection, which is usually based on simple mechanisms, compact design, inexpensive materials, and label free detection.
As advantages of LC based optical sensors, birefringent properties and extreme sensitivity to surface interactions place LC in a unique position in the sensor application. Due to its features such as the elastic long range interaction between LC molecules, high birefringence, low viscosity, and thin film technology, a LC based optical sensor exhibits specific advantages such as amplified signal, label free detection, fast response, portability, and low cost.
The fluidic nature of a liquid crystal promotes fast and highly sensitive response to external stimuli such as electric/magnetic/thermal field, chemical, and biological species. The highly anisotropic molecular structures and ordered molecular arrangements of a liquid crystal can generate extraordinary optical, electric, and magnetic effects. The combination of these characteristics makes LCs an ideal material for sensor applications. Today's ubiquitous LCDs are essentially LC based electric field sensors, where a LC molecule's orientation is rotated by an electric field triggering a macroscopically visible signal. As an electric field switches the LC away from the state defined by the alignment film, a change in optical response is produced. This interaction between the electric field and the liquid crystal is the general principle for the operation of most LC display devices. Many chemical/bio species can induce similar responses in LCs either by themselves or, more commonly, paired with specially tailored substrates and thin films. The prior art technique has many attractive advantages, such as simple and compact design, portable and even wearable devices, and label free detection; and thanks to mature LCD technology, the device can be very cheap and easy to manufacture.
As LCs become increasingly important for display, telecom, and biological applications, more complete control of alignment characteristics of the liquid crystal director would be advantageous. Many research groups have investigated homogeneous alignment layers, such as common polyimides, but it has proved difficult to control certain characteristics of the director alignment. For example, control of an anchoring transition for bistable devices and sensors, or control of the pretilt over the entire angular range from 0 to 90 degrees, are both difficult to achieve. The pretilt angle is an important parameter for characterization of an alignment film, which is the angle between the LC director and the film plane.
Another method for pretilt control involves the use of a heterogeneous alignment film, wherein the alignment of LC molecules results from the averaged interaction of regions that provide a different alignment preference. An example is a multilayer film proposed by de Gennes, Dubois-Violette (Sonin, A. A. The surface physics of liquid crystals. Gordon and Breach Publishers, 1995) and Hinov (Blinov, L. M. and Sonin, A. A. The interaction of nematic liquid crystals with anisotropic substrates. Mol. Cryst. Liq. Cryst. Sci., 1990. 179: p. 13-25) who predicted a “local Frederiks transition” of LC alignment on a crystalline surface covered with a thin amorphous film. The crystalline substrates considered imposed planar alignment of LCs via long range van de Waals (VDW) forces, while a short-range interaction between the LC and the top amorphous film favored homeotropic alignment. Later, Blinov and Sonin observed the anchoring transition of MBBA on a Langmuir-Blodgett film coated mica surface (See Wan, J. T. K.; Tsui, O. K. C.; Kwok, H.-S., and Sheng, P. Liquid crystal pretilt control by inhomogeneous surfaces. Phys. Rev. E, 2005. 72(2): p. 021711/1-021711/4; and Yeung, F. S.; Ho, J. Y.; Li, Y. W.; Xie, F. C.; Tsui, O. K.; Sheng, P., and Kwok, H. S. Variable liquid crystal pretilt angles by nanostructured surfaces. Appl. Phys. Lett., 2006. 88(5): p. 051910/1-051910/3). They found an abrupt planar to homeotropic anchoring transition as the LB film thickness increases, which can be denoted this as a Type I pretilt transition.
In contrast to the continuous layers, a spatially heterogeneous single layer alignment film has recently been investigated for example by Kwok (Estes, W. E.; Higley, D. P.; Auman, B. C., and Feiring, A. E., Liquid crystal displays of high tilt bias angles. U.S. Pat. No. 5,186,985, Feb. 19, 1993; and Creagh, L. T. and Kmetz, A. R. Mechanism of Surface Alignment in Nematic Liquid Crystals. Mol. Cryst. Liq. Cryst. Sci., 1973. 24(1&2): p. 59-68) for 0 to 90° degree pretilt control. Kwok mixed commercial vertical and planar aligning polyimides (PIs) and obtained thin films with a spatially uniform microphase separation. On the vertical/planar PI enriched domains the LC molecules align vertically/planarly. The LC director acquires an averaged pretilt in the bulk. The pretilt value is determined by the area ratio and the anchoring energy difference between the two PIs. This pretilt transition as the area ratio is changed is much more gradual than the abrupt Type I pretilt transition, and is denoted as Type II pretilt transition.
Moreover, the sensitivity of the LC sensor strongly relies on the surface condition on the substrate and the interaction between the LC and the surface features. When this interaction between LC and unsensed surface (i.e. surface before being exposed to target species) is very strong, it would be difficult or require a high level of target molecules to trigger the orientation change. Therefore, in order to increase the sensitivity, it is crucial to tune the interaction between a LC and unsensed surface just above the threshold requirement for the orientation change.
Advantageously, the present invention provides a liquid crystal (LC) device and methods of using and making the same. The device, when used in sensor applications, exhibits improved sensitivity; and when used in LCD applications, exhibits enhanced control over pretilt transition, among other merits.